Lasers produce an intense, coherent beam of light which is useful in many fields, including, for instance, medical surgery, fiberoptic communications, industrial heating and cutting, and radar. The term laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Although the acronym laser is specific to electromagnetic radiation in the visible spectrum (light), devices exist for producing radiation throughout the electromagnetic spectrum by stimulated emission of radiation. For instance, masers produce Microwave energy Amplification by the Stimulated Emission of Radiation.
Atomic lasers are the most common type of laser in use today. Atomic lasers amplify light at well defined frequencies which correspond to discrete electron energy transitions which occur within the atoms which comprise the lasing medium. For instance, an argon atomic laser can produce light only of a distinct wavelength corresponding to the possible electron energy transitions which may occur within an argon atom.
The present belongs to another class of lasers, known as free electron lasers. Free electron lasers do not depend on discrete energy transitions in atoms, but rather produce radiation through the interaction of free electrons in a magnetic field with an electromagnetic wave. A free electron laser can produce high powered, coherent radiation in a broad range of frequencies, and in theory throughout the entire electromagnetic spectrum dependent on proper matching of operating parameters. Further, while atomic lasers are typically characterized by efficiencies of less than 10%, free electron lasers are theoretically capable of efficiencies in excess of 50%.
In a free electron laser, a stream of free electrons is caused to travel through a vacuum at relativistic speeds (e.g., speeds exceeding of about 10% of the speed of light). The electrons are not tied to atoms as in atomic lasers. Because the electrons are free, they are not limited to particular energy level transitions and, therefore, can be caused to emit radiation essentially throughout the electromagnetic spectrum depending on various conditions. Dozens of schemes have been devised for amplifying electromagnetic radiation by means of free electron interaction.
For a free electron laser to produce electromagnetic radiation of more than negligible power, the electrons in the electron beam must be caused to physically bunch together. This is because the power radiated by a non-bunched stream of electrons is only the power of each electron multiplied by the number of electrons. However, Maxwell's equation dictates that if electrons are bunched together in a group much smaller than the wavelength of the emitted radiation, the power of the radiation is given by the power of each electron multiplied by the number of electrons squared.
Bunching can occur when a light wave traverses an undulating magnetic field because the spatial variations of the field and the electromagnetic wave combine to produce a beat wave (or interference pattern), which is called a ponderomotive wave. The ponderomotive wave propagates at less than the speed of light and thus can be synchronous with the electrons in the electron beam. Electrons which are synchronous with the ponderomotive wave are said to be in resonance with the ponderomotive wave and will thus experience a constant field corresponding to the portion of the ponderomotive wave with which it is traveling in synchronism. The details of the amplification of electromagnetic radiation from this interaction are not dealt with in detail here since detailed explanations can be obtained from other sources, such as the Encyclopedia Of Lasers And Optical Technology, Robert A. Meyers, Editor, Harcourt Brace Jovanovich, 1991 and references cited therein.
In short, proper matching of the velocity of the electrons to the velocity of the ponderomotive wave causes a resonance condition in which a very strong interaction between the electrons and the ponderomotive wave occurs causing electron bunching and stimulated emission of radiation at particular "resonant frequencies". Thus, a free electron laser can be configured to amplify electromagnetic radiation, at least theoretically, throughout the electromagnetic spectrum by proper matching of the energy (velocity) of the electrons and the undulating wiggler field (upon which the ponderomotive wave as well as a component of the electron velocity depend). Amplification is not unlimited, but reaches a saturation point at which radiation emission ceases growing and no greater power (energy per unit time) can be maintained in the laser. Increasing the length of the tube will not increase power once saturation is achieved.
In a free electron laser/maser, stimulated emission of radiation at a particular resonant frequency is induced by the interaction of three elements within an electron drift tube: 1) free traveling electrons, 2) an electromagnetic wave traveling in the same direction as the electrons, and 3) a transverse, undulating magnetic field (which may be produced by a magnetic wiggler). The resonant frequency is dependent upon both the periodicity of the wiggler magnetic field and the energy of the electrons as explained more fully below.
Free electron lasers frequently also employ an axial magnetic field parallel to the direction of propagation of electrons which prevents the electrons from dispersing due to their natural tendency to repel one another. The flux lines of the axial magnetic field are directed in the direction which will tend to reinforce the rotation of the electrons induced by the wiggler field. This axial field is termed the guide field.
The magnetic field produced by the wiggler is transverse to the direction of the electron beam and the guide field. Its signal is selected to add a transverse component to the velocity of the electrons, causing them to travel in helical gyration down the electron drift tube. The electromagnetic wave traveling in the same direction as the electrons should be of a resonant frequency of the system. The interaction of the electromagnetic wave of a proper frequency with the gyrating electrons causes stimulated emission of radiation t the resonant frequency leading to amplification of the wave.
The resonant frequency is dependent upon both the periodicity of the wiggler magnetic field and the axial velocity of the electrons. However, the axial and transverse velocities of the electrons are interdependent. Further, the transverse velocity of the electrons is a function of both the wiggler periodicity and the wiggler field strength. Accordingly, any of (1) the wiggler periodicity, (2) the wiggler field strength, and (3) the axial electron velocity can be adjusted to effect the desired resonant frequency where amplification can occur. (Amplification can also occur at any harmonic of the wiggler periodicity or any harmonic of the fundamental resonant frequency).
As mentioned, the guide field is parallel to the direction of propagation of the electrons in the electron beam. Its direction is typically selected dependent upon the direction of helical rotation of the electrons imparted by the wiggler field. When the electrons travel in a straight line parallel to the guide field, the guide field will have no effect on their motion. However, if the electrons have a velocity component transverse to the guide field, then the guide field will exert a force on the charged particles. Depending on the direction of rotation of the electrons, the guide field will either reinforce the rotation or oppose it. Typically, the guide field is oriented so that its flux lines are directed in the parallel direction which reinforces the helical rotation of the electrons. Thus, if the wiggler field imparts a clockwise rotation to the electrons when viewed looking in the direction of electron propagation (hereinafter termed right handed rotation), the guide field is directed so its flux lines are in the same direction as the direction of propagation of the electron beam. If the wiggler field produces a left handed rotation in the electrons, the guide field is directed so that its flux lines are opposite to the direction of propagation of the electrons so as to reinforce the rotation. Hereinafter these guide field directions, which depend on the electron gyration direction, will be referred to as the forward guide field direction.
A forward guide field leads to an increase in the transverse electron velocity compared to what it would be in the absence of an axial magnetic field, with potential benefits such as enhanced growth rate and efficiency. A reverse axial magnetic guide field would cause the electrons to have less transverse velocity. Thus, it was believed that use of a reverse guide field would reduce the growth rate of the laser because the transverse electron velocities would be reduced.
Free electron lasers typically operate at efficiency levels of less than 10%. Thus, less than 10% of the energy contained in the electron beam is converted into electromagnetic radiation. Further, as the desired output frequency increases, power requirements to obtain the necessary resonant frequency increases rapidly, making power requirements very great for free electron lasers working at the higher microwave frequencies and above.
It is an object of the present invention to provide an improved free electron laser.
It is a further object of the present invention to provide an improved free electron laser with an increased saturation level.
It is another object of the present invention to provide a higher efficiency free electron laser.